How can I make a hollow cylinder faces to be smooth but not rounded just smooth?
Smooth shading works by averaging the angle between two faces/edes/vertecies. This allows for smooth looks while keeping geometry low. 

There are multiple ways to retain sharpness in a model: 


Adding an edge split modifier. This will add additional geometry to mark edges sharp that are above a certain angle
Clicking on the "Vertex" tab on the right hand menu (it looks like a little updside-down triangle) You'll find an option to hit "Auto Smooth". This doesn't add any additional geometry and is my recommended way of going about this.
If you don't want to go with either of the two previous options then you can add a bevel modifier set to 0 width. This doesn't make a lot of sense and performs the same function as the edge split modifier.
You can mark individual geometry as "Sharp" by hitting CTRL + E and selecting "Mark Sharp"